1. Field of the Invention
This invention relates generally to a d-c arc welding apparatus, and more particularly to a d-c arc welding apparatus in which both an engine-driven welder and a battery-driven welder are provided and combined together.
2. Description of the Prior Art
In general, mobile engine-driven arc welders are used for welding operations at outdoor sites where no electric power is available. As such an engine-driven welder, an apparatus for converting the a-c output voltage of an alternator driven by an engine into d-c voltage via a rectifier for use as the drive power of the welder is publicly known. Such a welder commonly uses a gasoline engine of an output of 6-8 PS, with the welding output current being 140-170 A. For welders having a welding output current as high as 280 A, an engine having an output of 18-20 PS, such as a diesel engine is usually employed. Aside from the above-mentioned engine-driven welders, mobile battery-driven welders in which a plurality of batteries are connected in series to generate a welding output current of approximately 140 A are often used as d-c arc welders.
The mobile-type d-c arc welders mentioned above are useful for relatively small-current welding, as often found in construction sites and other work sites. With the progress of welding techniques and the increased size of structures being welded in recent years, demand for high-current welders of a welding output current of 280 A has been increasing to meet the increased need for high-current welding operations in construction sites. To have such a high welding output current, the conventional gasoline engine has to be replaced with a diesel engine, and the manufacturing process of the engines also has to be radically changed, making the manufacture of engines complicated or even impossible on the side of the manufacturers of gasoline engines. In addition, engine-driven welders involve offensive exhaust gas, noise and vibration caused by the engines, making it impossible to use in indoor sites and limited working space. In such work sites, battery-driven welders are more effective. Even in such applications, however, battery-driven welders cannot be operated for a length of time exceeding the capacity of the battery if no commercial power source is available, and the use of them is therefore limited to welding operations for a relatively short period. Furthermore, even when a high-output welder equipped with a diesel engine is installed in welding sites where high-current welding operations are expected, as noted earlier, high-current welding operations are not carried out at all times, but low-power welding operations are also frequently performed. In such cases, performing low-current welding with a high-power welder could lead not only to poor economy in terms of running costs, including the fuel cost of a diesel engine, but also to deteriorated working environment caused by noise and vibration.
FIG. 19 shows a circuit diagram of a conventional choppertype battery welder. Reference numeral 1 refers to a power source; 2 to a reactor; 3 to a swiching element, such as a transistor; 4 to a shunt resistor; 5 to a controller; 6 to a weld zone; 6-1 and 6-2 to welding output terminals; 7 to a commutating diode; and 8 to a photocoupler, respectively.
The reactor 2 is usually connected in series to the line to smooth current and prevent arc extinguishing.
When the transistor as the switching element 3 is turned on via the photocoupler 8 by the control of the controller 5, current flows in the direction shown by a solid-line arrow in the figure in a circuit consisting of the power source 1, the reactor 2, the shunt resistor 4, the transistor as the switching element 3, the earth and the power source 1.
When the transistor as the switching element 3 is turned off, the energy accumulated in the reactor 2 causes current to flow in the direction shown by a dotted-line arrow in the figure in a circuit consisting of the reactor 2, the shunt resistor 4, the weld zone 6, the commutating diode 7 and the reactor 2 to prevent arc extinguishing at the weld zone 6.
A welding output having a constant-current characteristic as shown in FIG. 21 is obtained by detecting a welding current flowing in the shunt resistor 4, and producing a PWM (pulse-width modulation) controlling signal in the controller 5 on the basis of the detected welding current so as to obtain the optimum current to control the transistor as the switching element 3 via the photocoupler 8.
With the conventional circuit configuration as shown in FIG. 19, in which a shunt resistor 4 for detecting current is connected in a loop of the reactor 2, the negative potential side of the shunt resistor 4 has to be isolated from the earth, making it impossible to use an earth line in the controller 5. As a result, the transistor as the switching element 3 has to be driven by an insulated circuit element, such as the photocoupler 8.
This necessitates an expensive floating charge power source for the controller 5 and the use of an insulated circuit element to drive the switching element 3, increasing the number of circuit components.
With the circuit configuration shown in FIG. 20, the power source 1 of the battery welder not only supplies power to the welding circuit, but also prevents the battery 11 from being charged, and overvoltage from being input to the power source.
A commercial power source 9 is fed to the primary side of a transformer 15 via a no-fuse breaker 13 and a relay 14, and the voltage generated in the secondary side of the transformer 15 is rectified by diodes 16 and 17 to charge the battery 11 and supply d-c power to the welding circuit.
The battery must be charged in a trickle-charge mode to stabilize the specific gravity of the battery solution even after the specified voltage is reached. Trickle charging is performed for a predetermined period using a timer by reducing the secondary-side output voltage by changing over the tap of the primary winding of the transformer 15.
Overvoltage is prevented from being input to the commercial power source 9 to cause damage to the battery 11, the transformer 15, and the diode 16, when 100-V power is connected inadvertently to a 200-V power source, as often experienced in Japan, by providing a no-fuse breaker 13, which is also used as a main switch, in the primary side of the transformer 15.
In the conventional battery charging practice where trickle charging is performed by changing over the tap of the primary winding of the transformer 15 using the relay 14, reliability could be substantially lowered by the contacts of the relay 14, and provision of an intermediate tap in the primary side of the transformer 15 could lead to an increase in the equipment cost.
The above-mentioned overvoltage protection method using the no-fuse breaker 13 which also serves as a main switch requires a certain time until the no-fuse breaker 13 is actuated, and could cause damage to the transformer 15, the diodes 16 and 17, etc. when the user is not aware of overvoltage even after the equipment is put into operation and repeatedly turning on the no-fuse breaker 13.
In a battery-driven welder controlled by electronic circuits, a transistor as the switching element 3 is used for controlling welding current. The switching element 3 for controlling large current is mounted on a heatsink to dissipate heat. In addition, a fan is normally used to increase cooling efficiency.
This fan is driven by a battery 11 which is a power source for the welder.
The control of the rotation/stop of the conventional welder fan is interlocked with the turning on and off of the main switch.
With the construction in which the control of the rotation/stop of the fan is interlocked with the main switch, as found in the conventional type, the fan is kept operating even when welding operation is not carried out, causing much loss of energy. If the main switch is inadvertently left turned on, the battery could exhaust.